feat(*): add some missing `coe_*` lemmas (#6697)
* add `submonoid.coe_pow`, `submonoid.coe_list_prod`,
`submonoid.coe_multiset_prod`, `submonoid.coe_finset_prod`,
`subring.coe_pow`, `subring.coe_nat_cast`, `subring.coe_int_cast`;
* add `rat.num_div_denom`;
* add `inv_of_pow`.
